1. Field of the Invention
The present invention relates to a device for controlling rotor vibration. In particular, the present invention relates to a device utilizing active magnetic bearings to control and isolate rotor vibrations from the housing and its mountings.
2. Background of the Prior Art
It is well known in the art to support a rotating shaft using electromagnetic coils with an active feedback system. The obvious advantage of such a system is the elimination of a lubricant or lubrication system. Another advantage is the ability to control the stiffness and damping support characteristics which in turn permits controlling the vibrational characteristics of the rotor. In certain applications, the ability to control and isolate rotor vibrations is more important than eliminating a lubrication system or lubricants.
One problem in using active magnetic bearings is preventing a catastrophic failure due to a control system malfunction. If the control system malfunctions, the rotating member of the active magnetic bearing would make contact with the electromagnetic stator resulting in severe damage. One way to avoid such damage is by using a "catcher" or an auxiliary bearing to catch the rotor before contact is made either between the clearance type seals or the magnetic bearings themselves.
Accordingly, an active magnetic bearing is typically protected by an auxiliary bearing. However, auxiliary bearings usually suffer from a short lifetime and are used to support the rotor until they stop. Oil type sleeve bearings are not employed as auxiliary bearings since there is insufficient time to activate an oil supply system and to supply the oil during an active magnetic bearing failure
Rolling element bearings are most often used as the "catcher" or auxiliary bearing. These bearings have a clearance between the shaft and bore to prevent contact during operation of the active magnetic bearing. When the active magnetic bearing fails, the shaft drops onto the bore of the auxiliary bearing inducing skidding of the rolling element and reducing life due to the loose bore. If the rotor is highly unbalanced, a violent whirl can occur resulting in even greater associated damage.